This invention relates to an automatic switchover valve commonly used for switching from a bank of gas supply cylinders which has had its pressure depleted below a predetermined pressure to a fresh bank at full pressure for supplying the gas to an end use.
Such valves are automatic and many operate on a known principle where the switching occurs because of a sensed differential in pressure between the full pressure cylinder bank and the depleted cylinder bank. A typical valve of this type is shown and described in British Patent No. 970,307, where the "shuttle" has two areas at each end thereof, a smaller area which enters into the inlet valve openings and a larger area within the valve housing. The pressure on the "in use" side of the valve acts upon the larger or full area of the shuttle to maintain the valve in that position. As this pressure is reduced due to depletion of the gas, a point is reached where the high pressure on the full cylinder side of the valve, acting upon only the smaller end area of the shuttle, forces the shuttle toward closing the in-use side. The movement of the shuttle begins fairly moderately until the smaller area is withdrawn from the full cylinder inlet opening, at which time, the full pressure then acts upon the full end area of the shuttle and it is rapidly propelled to close the in-use inlet and open the full cylinder inlet to supply gas from that full supply pressure to an outlet in the valve.
By use of the two area concept, the depleted gas cylinder can be replaced by a full cylinder, which then acts only upon a smaller, reduced area than the area upon which the in-use pressure acts. Thus, the valve will not shuttle when the new tanks are installed, but only when the in-use tanks are depleted to the predetermined pressure, i.e. the predetermined pressure differential again exists.
One difficulty with this type of valve is the possible build-up of pressure in closed or trapped areas within the valve when the shuttle moves toward the depleted side. One method used to avoid the problem of pressure build-up is to provide a venting of any such areas to the depleted gas cylinders as described in U.S. Pat. No. 3,633,606.
A further difficulty is in providing adequate sealing for the shuttle, that is, for both the smaller area end portion and the larger area end portion of the shuttle. Both areas must be sealed around their periphery against internal valve surfaces to prevent leakage which may also cause a premature shuttle. The seals in present valves can easily wear through the fast shuttling action or by the introduction into the valve of minute particles of dirt or other foreign material. The wearing can be particularly a problem where seals are embrittled by cold weather, when used outdoors, or where piping gases which cause a refrigerant effect such as nitrous oxide. The cold, hardened seals are easily harmed by sliding against sharp edges or orifices in the shuttle.
Other difficulties include alignment or switching and may employ springs or other mechanical means to assist in the shuttling. Such remedies are, however, costly, difficult to manufacture, and lose their preciseness through a change in the spring constants through wear or even possible spring breakage.